The present invention relates generally to the production of coilable metallic tubing, and in particular to the production of continuous lengths of carbon or stainless steel coilable tubing in various lengths up to, and exceeding, 20,000 feet.
Coilable metallic tubing is produced from flat metal stock which is typically shipped in 4 feet widths and in lengths of 4,000 feet. This stock is then sliced into widths approximately 4 inches wide to form "strips" which are 4000 feet in length. These strips are unspooled into an accumulator. An accumulator is a device which periodically quickly draws flat metal strips from a supply coil into one end while slowly and continuously feeding the metal stock out an opposite end. From this opposite end the metal strip is relatively slowly but continuously fed into a tube forming apparatus now known in the art.
As noted, a flat strip is preferably periodically drawn from the supply coil into the accumulator at a relatively high rate of speed for a short time. This periodic accumulation of flat strip from the supply coil results in periods of from 9 to 13 minutes of inactivity while the strip is slowly and steadily fed from the output end of the accumulator. This period of inactivity provides sufficient time to replace the supply coil spool which is depleted with another full supply coil. The new supply coil may be brought into position, and the leading end of the new flat strip may be tack welded to the trailing end of the strip from the depleted supply coil. This prior art arrangement enables the tube forming apparatus to operate continuously without interruption due to supply coil replacement. In addition, the cumbersome and time consuming rethreading of the tube forming apparatus is eliminated.
Currently, steel mills have the capability of producing master coils of flat metal strip of up to 4,000 feet in length. In the past, although a trailing end of the strip from a depleted supply coil was tack welded to the leading end of an additional supply coil in order to keep the tube forming machinery in continuous operation, an inferior joint was formed every 4,000 feet. In the past, after tubing from one entire supply coil had been produced, the short section of tubing formed on both sides of the tack welded union of the two lengths of flat strip was cut out of the tubing string and discarded. The result has been that continuous lengths of only about 4,000 feet of coilable metallic tubing have been produced.
These 4,000 foot long lengths of coilable tubing string then have been transported to a furnace, heat treated, and placed in inventory. When a length of up to 20,000 feet of coilable metallic tubing has been required, ends of the individual 4,000 foot lengths of tube have been butt-welded together to form the required length of tubing.
Although providing a degree of satisfactory performance, these butt-welded joints have been the source of significant concern to persons skilled in this art. In some situations where coilable metallic tubing is required, and particularly in oil field applications, the tubing must be able to sustain very high pressure and repeated coiling and uncoiling stresses. Therefore, the butt-welds joining the individual 4,000 foot lengths of tubing must be of very high quality. In order to check the integrity of the butt-welds, each butt-welded connection is subjected to x-ray nondestructive testing procedures. However, because a butt-weld is a circumferential weld, each butt-welded joint must be x-rayed at an angle in order to avoid a shadowing effect whereby the front of the welded joint shadows the back of the joint. Further, because a butt-weld is a circumferential weld, the butt-welded joint must be x-rayed a number of times at slightly different rotational positions about the longitudinal axis of the tubing in order to compensate for the radial shadowing effect at opposed diagonal regions around the tube.
Butt-welding lengths of coiled tubing also may weaken the overall strength of the tubing string. When ends of two lengths of tubing are butt-welded together, the butt-welding operation creates a heat-affected zone in the tubing on each side of the weld. The tensile strength of the tubing string in these heat-affected zones will typically be 90% or less of the tensile strength of a typical section of the previously heat-treated tubing string. Further, the heat-affected zone on each side of the butt-weld is entirely circumferential, therefore creating two potentially weak zones, one on each side of each butt weld.
When the overall length of tubing string is spooled off of a large transport reel and inserted downhole into a well, and is then removed from the well and spooled back onto the reel, each butt joint experiences significant tensile forces and bending moments. In the past, the tubing has occasionally failed at a heat-affected zone adjacent the butt-weld.
Whereas known techniques facilitate the grinding away of excess weldment around the outside diameter of the butt-weld, the circumferential accumulation of weldment around the inside diameter of the butt-weld cannot as readily be removed. As a result, with prior techniques, a bead of weldment reduces the inside diameter of the tubing at the location of the butt-weld. This reduced diameter region may interrupt smooth flow inside the tubing and may interfere with the introduction of maintenance devices referred to as "pigs" which are pumped through the tubing or of measuring instruments which may be lowered by gravity through the tubing.
For example, coilable tubing is sometimes used to transport abrasive fluids at high pressures downhole into an oil well. When these fluids encounter the circumferential bead or ring of weldment built up around the inside diameter of the butt joint, a turbulence is formed down stream of the butt-weld. This turbulence may cause the abrasive fluids to abrade or cut through the wall of the tubing string from the inside, thereby causing tubing failure. Also, in the past, occasionally measuring instruments and pigs became lodged on the built up circumferential ring of weldment around the inside diameter of the butt-weld.
Thus, the combined tubing strings known in the prior art have demonstrated various disadvantages which heretofore have not been overcome.